The present application relates to capacitive microphones with a sealed gap between the capacitor's conductive plates, and more particularly to capacitive Micro-machined Electro-Mechanical Systems (MEMS) microphones with a sealed gap for receipt of air-mediated sound.
Note that the points discussed below may reflect the hindsight gained from the disclosed innovative scope, and are not necessarily admitted to be prior art.
Microphones in consumer devices generally comprise pressure compensated MEMS microphones and pressure compensated electret microphones. An overview of pressure compensated microphones—that is, microphones which do not have a sealed gap—is provided below.
FIG. 1 schematically shows a cross-section of an example of a pressure compensated MEMS microphone 100. As shown in FIG. 1, a pressure compensated MEMS microphone 100 comprises an acoustic sensor 102 fabricated on a semiconductor substrate 104, the acoustic sensor 102 comprising a moveable, suspended membrane 106 (a vibrating plate) and a fixed sensor back plate 108. The back plate 108 is a stiff structure comprising perforations 110 that allow air to easily move through the back plate 108. Both the membrane 106 and the back plate 108 are connected to the substrate 104. The membrane 106 is located between the back plate 108 and the substrate 104, with a cavity 112 (a “gap”) between the membrane 106 and the back plate 108. The perforations 110 enable pressure compensation of the gap 112, that is, they equalize the pressure on each side of the back plate 108. The membrane 106 is suspended over a front chamber 114 formed in the substrate 104.
The vibrating plate in a microphone can be called a membrane or a radiation plate, depending on the ratio between the radius and thickness of the membrane or radiation plate, as further described with respect to FIG. 3.
The substrate 104 is mounted on a carrier 116, which can be, for example, a lead frame or a printed circuit board. There is also a back chamber 118, which is surrounded by the carrier 116 and an enclosure 120 (e.g., a metal casing). An integrated circuit 122 for charging electrodes attached to the membrane 106 and the back plate 108, and for the interpreting the signal produced by the acoustic sensor 102, is coupled to the membrane 106 and the back plate 108 by wire bonds 124. A soldering pad 126 coupled to the integrated circuit 122 enables external input to and output from (e.g., power and signal, respectively) the microphone 100.
The membrane 106 is a thin solid structure made of a compliant (not stiff) material, such as a perforated solid material suitable for micromachining, that flexes in response to changes in air pressure caused by sound waves passed by the perforations 110 in the back plate 108. The membrane 106 does not fully seal the gap 112. Also, perforations in the membrane 106 (not shown) increase the membrane's 106 responsiveness to air-mediated sound waves by reducing membrane 106 stiffness (increasing flexibility), and by helping to equalize pressure on both sides of the membrane 106 (the side facing the back plate 108 and the side facing the substrate 104). As described above, the perforations 110 in the back plate 108 enable pressure compensation of the gap 112. In pressure compensated MEMS microphones 100 (and similarly in pressure compensated electret microphones 200, described below), the air pressure in the gap 112 is equal to the ambient static pressure, that is, the atmospheric pressure (thus the description “pressure compensated”). A pressure compensated gap 112 enables a more flexible membrane 106, because a static pressure difference between the gap-facing and substrate-facing sides of the membrane 106 is reduced. This means that there is effectively no static force against the membrane 106 due to air pressure.
The “ambient” is the medium (acoustic environment) through which acoustic waves are conducted to intersect a membrane, causing the membrane to vibrate, resulting in a signal being emitted from the microphone. For example, in microphones included in smartphones, the relevant ambient will generally be the atmosphere (air). As used herein, an “airborne” microphone is defined as a microphone for which the primary intended ambient is air.
FIG. 2 schematically shows a cross-section of an example of a pressure compensated electret microphone 200. An electret is a stable dielectric material with a permanently embedded stable electric dipole moment—that is, a permanently polarized piece of dielectric material. An electret microphone is a type of electrostatic capacitor-based microphone which uses an electret, and can thereby avoid using a polarizing power supply (used in a MEMS microphone 100 to apply charge to electrodes).
As shown in FIG. 2, an electret microphone 200 comprises an acoustic sensor 202, which in turn comprises an electret membrane 204 (e.g., a polymer electret membrane 204). A front chamber 206 is located on a front chamber 206 side (a first side) of the electret membrane 204. The front chamber 206 side of the electret membrane is electroded, and is clamped to a metal washer 208 at the electret membrane's 204 rim. The electret membrane 204 is separated from a back plate 210 to create a gap 212 on a gap 212 side (a second side) of the electret membrane 204. A constant gap 212 height is maintained by, for example, plastic washers 214. The back plate 210 comprises perforations 216 so that the gap 212 is pressure compensated. An amplifying transistor 218 is fixedly coupled to a carrier 220 (e.g., a lead frame or printed circuit board), and the amplifying transistor's 218 gate pin is coupled by a wire 222 to the back plate 210. The connection between the amplifying transistor 218 and the back plate 210 conveys received signal from the acoustic sensor 202 to the amplifying transistor 218. The amplifying transistor 218 interprets the signal produced by the acoustic sensor 202. The carrier 220 is coupled to the back plate 210 by a casing 224 (e.g., plastic casing). The carrier 220 is also fixedly coupled to a housing 226 (e.g., a metal housing), which holds the carrier 220, the casing 224, and the acoustic sensor 202. This coupling also electrically connects the electret membrane 204 and a source lead 228 of the amplifying transistor 218. A hole 230 in the housing 226, located proximate to the front chamber 206, gives acoustic waves access to the electret membrane 204. The hole 230 and the front chamber 206 are covered by a dust cover 232, which does not seal the electret microphone 200. That is, air, as well as humidity and other contaminants, can access the interior of the electret microphone 200. Contamination can be mitigated, but not prevented, by the dust cover 232. The transistor 218 is located in a back chamber 234. The back chamber 234 is also proximate to the back plate 210 on a side of the back plate 210 distant from the gap 212. To maintain pressure compensation, the back chamber 234 is not sealed. Access to the source lead 224 and a drain lead 236 of the amplifying transistor 218 are provided at an outer surface of the carrier 220 (a surface distant from the back chamber 234) to enable external electrical connections for signal acquisition.
MEMS microphones 100 and electret microphones 200 detect sound by placing a fixed charge across the gap 112, 212, and measuring voltage variations caused by changes in the capacitance between the membrane 106, 204 and the back plate 108, 206 as a result of the membrane 106, 204 flexing in response to sound waves. MEMS microphones 100 apply the fixed charge using a bias voltage, and electret microphones 200 induce a fixed charge using an electret.
Typically, MEMS microphones 100 used in mobile phones are biased at 10 volts to 14 volts DC, generated using voltage doubler circuits to produce the appropriate voltage from a battery supply outputting 1.8 volts to 3.6 volts.
Typical electrets used in microphones are made of dielectric materials such as polymers used as membrane 204 material, or silicon oxide or silicon nitride in the back plate 210. Electrets can trap electrical charge in their bulk material or on their surface. Circuits including an electret are generally terminated using a terminating impedance. When the surfaces of an electret layer are properly electrically terminated, the trapped charge can yield, for example, a total charge corresponding to (which can be modeled as) a bias voltage of 150 to 200 volts polarizing the gap 212.
As discussed, pressure compensation means that the gap is open to ambient air in order to equalize gap pressure with ambient atmospheric pressure. A pressure compensated gap is therefore vulnerable to contamination by dirt, humidity or other foreign matter carried by the air that moves to and through it. Contamination of the gap can compromise microphone performance due to clogged gap vents, back plate perforations, and/or membrane holes, which cause noise. Membrane hole contamination reduces membrane compliance, which corresponds to a loss in microphone sensitivity. Also, material buildup in the gap can lower gap height, also lowering microphone sensitivity.
Signal-to-noise ratio (SNR) is the main competitive performance issue in the commercial microphone market, which encompasses microphones for devices such as smartphones, in-ear headphones and hearing aides. Typically, the SNR of commercial MEMS microphones ranges between 55 and 65 dB for a sensor area of approximately 1 mm2. In microphones, SNR is measured when the input acoustic signal level is 94 dBA. The unit dBA refers to A-weighted decibels, which accounts for the human ear's different perception of loudness at different frequencies.
SNR is defined as the ratio of: the root-mean-square (rms) voltage across the terminals of the microphone, when the microphone is placed on a rigid baffle and a free field pressure wave of 1 Pa rms amplitude at 1 kHz frequency is incident on the microphone; to the rms voltage across the terminals of the microphone, filtered using A-weighted filters, when the microphone is completely isolated from any sound sources, such as in an anechoic chamber. The sound level at 0 dBA, which corresponds to about 20 μPa rms, is accepted as the hearing threshold of the human ear (though clinically measured threshold levels are much louder). The maximum possible SNR is about 94 dB, because the inherent noise induced by acoustic radiation physics (the radiation resistance, described below, which provides a generally-applicable noise floor) is about 0 dBA in a microphone with 1 mm2 area.
A rigid baffle is an infinite, perfectly reflecting surface around the boundary of an acoustic aperture of a microphone. If a microphone is mounted on a rigid baffle, the incoming acoustic wave will create twice the free field pressure on the microphone's vibrating element that it would in empty space.
Noise in a microphone, which reduces the maximum possible SNR of the microphone, predominantly comes from one of three sources: radiation resistance of the membrane; mechanical losses caused by molecular friction in the material of vibrating parts, and/or by macroscopic friction of mechanical parts in the microphone moving against each other; and in pressure compensated microphones, mechanical losses caused by fluid friction, including the friction of air moving through perforations (holes) in a membrane or substrate, and the squeezed film friction effect in the gap. There can be other losses, such as electrical energy loss from dielectric loss in the insulator layer. Some pressure compensated MEMS microphones have a noise floor of about 30 dBA, with pressure compensation contributing most of this noise. The noise floors in pressure compensated electret microphones are generally higher than in comparable MEMS microphones.
Radiation resistance is the real component of radiation impedance (a complex number). Radiation impedance relates to Newton's third law of motion: every action has a reaction of equal magnitude and in the opposite direction. A transmitting acoustic transducer (such as a loudspeaker) applies a force onto the medium (pushes the medium, such as air, to and fro) at its aperture during transmission. The medium also exerts a reaction force on the transducer surface. The reaction force is equal to the product of the velocity of the transducer surface (the aperture) and the radiation impedance. Radiation impedance is a complex number with two components: radiation resistance (the real component) and radiation reactance (the imaginary component). Part of the reaction force, corresponding to the radiation resistance, generates acoustic waves, which radiate out from the aperture into the medium. The energy comprising the radiated acoustic waves (corresponding to the radiation resistance) is lost with respect to the transducer (the transducer does not recover the energy used to create the acoustic waves).
Acoustic transmission and acoustic reception are reciprocal phenomena. Therefore, radiation impedance is also present in acoustic reception (microphones). Radiation resistance is a source of noise in acoustic reception. The noise generated by radiation resistance is the noise floor of a 100% efficient microphone with no other sources of mechanical or electrical energy loss.
When an acoustic wave is incident on the microphone membrane, the acoustic field energy is included in the transduction and a force is applied on the membrane surface, which moves the membrane. The reaction force of the membrane, applied onto the medium (the ambient), is equal to the product of the radiation impedance and the velocity of the membrane. The incident acoustic energy is first partly dissipated by the resistive part of the radiation impedance. Remaining energy is then available to the transduction mechanism (that is, acoustic reception in a microphone). Radiation resistance is an energy dissipative factor in transduction, and therefore generates noise during reception.
The squeeze film effect refers to two consequences of air periodically squeezed between a vibrating membrane and a static substrate: (1) increasing air pressure forces air to escape from the gap through available outlets, e.g. holes, causing friction, which dissipates (loses) energy; and (2) increasing air pressure in the gap increases the temperature of the temporarily compressed (squeezed) air (following Gay-Lussac's Law), which causes energy loss by converting mechanical energy into heat.
Some typical integrated commercial MEMS microphones used in mobile phones are operated with a dc bias voltage of 10-14 volts, with an approximately 28-30 dBA noise floor in their audio bandwidth. This amount of self noise corresponds to an SNR of 66 dB or less at the transducer output before pre-amplification, when the incident signal level is 1 Pa. Such commercial MEMS microphones typically have about −38 dB re V/Pa maximum OCRV (open circuit receive voltage) sensitivity.
A Capacitive Micromachined Ultrasonic Transducer (CMUT) is a capacitive transducer. CMUTs can be used to transmit and receive ultrasonics. CMUTs have a wide bandwidth in water and in a frequency range near their first (lowest) resonance frequency. Microphones generally have many resonances. At a resonance, the amount of applied force, external pressure or electromechanical force required to induce high-amplitude vibration of the membrane is reduced. Ultrasonic transducers (such as CMUTs) are usually operated near their first resonance frequency. This enables the transducers to be highly sensitive; however, for efficient transmission and/or reception to be maintained, the transducer will have either a narrow operation bandwidth, or increased internal loss and consequent increased noise (lower SNR). Internal loss is power loss, and is the sum of power lost through mechanical and electrical energy loss mechanisms other than radiation resistance.
In some examples, CMUTs can have a pressure compensated gap, resulting in a compliant radiation plate and a relatively wide bandwidth. In some examples, CMUTs can have a sealed gap, resulting in low internal loss (in some examples, less than their radiation resistance in air). CMUTs are typically characterized as receivers when operated at a resonance frequency, and as microphones when operated off-resonance. A sealed gap can contain a sealed-in gas, or a vacuum (a “vacuum gap”). Internal loss in CMUT transducers is typically small with respect to the noise introduced by radiation resistance—small enough to be difficult to accurately measure. In some examples, losses and radiation impedance in sealed gap airborne CMUTs generate about 0 dBA in the audio bandwidth, which is slightly more than the noise contribution of the CMUT's radiation resistance in a 1 mm2 microphone operated off-resonance in an audible range (generally, about 10 Hz to 20 kHz).
A pressure compensated MEMS microphone comprising a transducer, sealed membranes and a sealed volume is disclosed by U.S. Pat. No. 6,075,867.
An integrated and programmable microphone bias generation system is described by U.S. Pat. No. 8,288,971.
An implantable microphone which uses a housing to hermetically seal the microphone is described in U.S. Pat. No. 9,451,375. This microphone compensates for noise artifacts caused by the housing by using two highly compliant parallel membranes, compliance of the membranes being enhanced by respective pressure compensated gaps.
An implantable microphone which uses a perforated membrane for pressure compensation is described in U.S. Pat. No. 7,955,250. The perforation in the membrane makes the membrane more compliant, and thus increases sensitivity. U.S. Pat. No. 9,560,430 also describes a microphone with a perforated membrane.
A microphone module which uses vents to enable pressure compensation, and for driving water out of the system, is described by U.S. Pat. Pub. No. 2015/0163572.
A pressure compensated microphone module for a phone watch that uses a hydrophobic plate covered by an “impermeable” membrane—which allows passage of gasses—to enable pressure compensation, and to keep water out of the microphone, is described by Pat. Pub. No. 2001/0019945.
Some microphones use hydrophobic and/or oleophobic materials to cover microphone components to protect them from fluids. For example, a microporous composite material containing polytetrafluoroethylene (PTFE) is described in Pat. Pub. No. 2014/0083296 for use in filters, vents or protective membranes. PTFE is gas permeable such that it can both be used as a protective membrane and enable pressure compensation. A hydrophobic mesh (umbrella-shaped, covering an acoustic port), is described in U.S. Pat. No. 9,363,589. However, PTFE, hydrophobic mesh, and other methods of “waterproofing” microphones with pressure compensated gaps will generally degrade performance (due to isolation of sound-detection membranes from sound sources), and will fail to protect transducers from water given a relatively small static pressure difference between the external environment (e.g., immersion in water at a depth of a meter) and the gap, or given repeated submersion.
A MEMS microphone with a piezoelectric (rather than capacitive or electret) membrane, which can be covered by a Parylene film for waterproofing, is described in U.S. Pat. Pub. 2014/0339657. Piezoelectric MEMS microphones are fabricated using different production processes than capacitive microphones.
The inventors endeavor to disclose new and advantageous approaches to a capacitive MEMS microphone with a sealed gap, and methods for designing such microphones, as further described below.